The experiments proposed here are designed to analyze the process of gastrulation in the mouse embryo. The parameters to be studied, in both normal and mutant embryos, include (1) the embryonic growth patterns, (2) cytoskeletal elements, and (3) extra-cellular matrix production. The methods of analysis include light and electron microscopy, immunomicroscopy, and autoradiography. The objective of these studies is to define the critical cellular processes involved in mammalian gastrulation. The use of the developmental lethal mutations tw5, TOR, and Torl, which are all associated with the T/t-complex of the mouse, will allow a detailed analysis of the critical developmental processes. Mutations at the T/t locus are thought to affect development via abnormalities at the cell surface. The experiments proposed here will produce analogies between these specific cell surface defects and the cellular process each affects.